Ok, you want to stream to Facebook live.
The easiest setup (and probably the only one you are technically able to achieve) is this: you use one OBS running on a machine on a central location that directly captures all your sources. This machine streams directly to Facebook. No nginx, no intermediate server or service.
What are your original sources?
- the pastor with his speech (audio)
- the video of the pastor holding his speech (video)
- worship music (audio)
- worship audio (whatever that is - is this the voices of the crowd in the church?) (audio)
- most church services add text overlays with verses and song texts
You need to capture these sources and feed them to OBS.
I don't know what you intend to capture with your cell phones, but usually you use simple cameras to capture video, simple microphones to capture speech, and often some computer-generated audio in addition.
Cell phones are not suitable capture devices if you need to composite your stream from multiple sources.
So choose a streaming PC capable of running OBS, with a CPU+GPU strong enough to encode the video stream. Connect that machine to the internet with a good enough upload bandwidth (probably 5000-8000 kbit/s).
Get one or more external mics and get one or more external cameras with cables long enough to physically connect to your streaming machine. The cameras can be simple webcams. You can connect more than one cameras. Many also use DSLRs with hdmi capture cards, but that's an advanced topic. Don't use cell phones as remote mics or remote cameras.
Connect the camera(s), connect the microphone(s), connect the soundboard directly to the streaming machine. Add additional sources to OBS, for example text sources for the verses. Create appropriate scenes in OBS and create appropriate audio mixes in OBS. Stream directly to Facebook.